1. Field of the Invention
The present invention relates generally to an apparatus and a method for calibration of a wireless transmitting apparatus. In particular, the present invention relates to an apparatus and a method for calibration of a wireless transmitting apparatus in an Orthogonal Frequency Division Multiplexing (OFDM) system.
2. Description of the Related Art
A fourth generation (4G) communication system such as wireless LAN (WLAN), and Digital Multimedia Broadcasting (DMB) requires large capacity data transmission, and thus it adapts an OFDM scheme in order to transmit high speed data of wideband. The OFDM scheme divides a using bandwidth into a plurality of subcarrier for transmitting data. The OFDM scheme divides a serial data row into N parallel data rows and transmits in each divided subcarrier, the data, so as to increase efficiency of the data.
In order to improve communication quality in a communication system, there is a method that a beam from an antenna is set towards a subscriber's direction by controlling a transmission power between a base station and a terminal or by controlling a horizontal direction and a tilt angle of a smart antenna of a base station. The method of controlling a horizontal direction and a tilt angle of a base station antenna is known to be more efficient than the scheme of controlling a transmission power because it enhances a Signal-to-Noise Ratio (SNR) by increasing a gain of the antenna for improving communication quality.
In order to precisely perform the method of increasing a gain of the antenna for improving communication quality, a delay and phase characteristic of a transmitter of each base station should be equalized. Therefore, a transmission calibration is performed for equalizing the delay and phase characteristic.
A transmission calibration is a method where a delay and phase characteristic of each path is previously calculated and corrected so that characteristics of signals outputted by a plurality of antennas cannot be varied according to a delay and phase difference of each Radio Frequency (RF) path.
Conventional calibration methods calculate a transmission function by comparing an additional reference signal with a signal output through a main path. A calibration signal transmitted by the conventional calibration method is an interference wave, and if a high-level calibration is performed, an interference signal factor is increased, and thus receiving sensitivity of a terminal may be deteriorated.
In addition, the calibration signal is an interference wave, and thus, a status of a transmit signal can be deteriorated according to a ratio of a transmitting data signal to the interference signal. Accordingly, an output power should be increased when the transmit signal has to be demodulated to have a signal quality requested by a receiver. In light of such drawbacks, the number of transmitter antennas or receivers should be reduced.
A conventional calibration apparatus comprises an additional antenna or a signal generator, and thus a manufacturing cost is increased and the structure of the calibration apparatus becomes complex.